1. Field of the Invention
The present invention relates to a receiver, a receiving method, and a radio communication system that are used in communication realized by MIMO, and more particularly to a receiver, a receiving method, and a radio communication system in which MIMO signals are demodulated by means of maximum likelihood detection (MLD) from signals that are received using a plurality of reception antennas.
2. Description of the Related Art
Radio communication methods in next-generation mobile communication demand high-speed data transmission. MIMO (Multiple Input Multiple Output) multiplexing is receiving attention as a technology for realizing high-speed data transmission. MIMO is a technology for transmitting a plurality of signals from a plurality of transmission antennas at the same frequency and the same time, receiving these signals using a plurality of reception antennas, and then demodulating (implementing signal separation of) the plurality of signals.
FIG. 1 is a block diagram showing the configuration of a typical radio communication system that uses MIMO. In this case, the number of transmission antennas is M (where M is an integer equal to or greater than 1), and the number of reception antennas is N (where N is an integer equal to or greater than 1).
Referring to FIG. 1, the radio communication system includes transmitter 81 and receiver 82. Transmitter 81 has a plurality of transmission antennas 831-83M, and receiver 82 has a plurality of reception antennas 841-84N.
Transmitter 81 transmits differing signals from each of a plurality of transmission antennas 831-83M at the same time and at the same frequency. Receiver 82 uses a plurality of reception antennas 841-84N to receive the signals that have been transmitted from transmitter 81 and, from these received signals, demodulates M signals by means of a signal separation process. According to this radio communication system, increasing the number of signals that are simultaneously transmitted and received in proportion to the number of transmission antennas enables the realization of high-speed data transmission without increasing the transmission bandwidth.
On the other hand, DS-CDMA (Direct Sequence-Code Division Multiple Access) is widely used as a mobile communication radio access method.
In DS-CDMA, time-spreading a transmission signal by a particular code can effectively reduce the interference of other cells in a multi-cell environment and enable one-cell repetition. In addition, separating multipath signals by means of despreading and then combining these multipath signals (rake combining) can obtain the path diversity effect.
In recent years, the possibility of enabling even higher-speed data transmission through the application of MIMO multiplexing to DS-CDMA (CDMA MIMO multiplexing) is being investigated. Various methods have been proposed as the signal separation process in CDMA MIMO multiplexing, examples including: Minimum Mean Square Error (MMSE), Vertical Bell Labs Layered Space-Time (VBLAST), and Maximum Likelihood Detection (MLD).
MMSE is a method in which interference from transmission antennas other than the target transmission antenna are suppressed by a linear filter. VBLAST is a method in which MMSE and successive interference cancellation of transmission antenna signals are repeated (for example, refer to A. Adjoudani, E. C. Beck, A. P. Burg, G. M. Djuknic, T. G. Gvoth, D. Haessig, S. Manji, M. A. Milbrodt. M. Rupp, D. Samardzija, A. B. Siegel, T. Sizer, H. C. Tran, S. Walker, S. A. Wilkus, and P. W. Wolnianski, “Prototype Experience for MIMO BLAST over Third-Generation Wireless System,” IEEE J. Select. Areas Commun., Vol. 21, no. 3, pp. 440-451, April 2003 (Document 1)). MLD is a method of generating replicas of all transmission antenna signals and using these replicas to select the most likely transmission antenna signal.
MLD has superior characteristics to MMSE and VBLAST but has the disadvantage that the increase in the number of transmission antennas and the number of modulation multivalues brings with it an exponential increase in computation load. As a solution to this drawback, a reduced-computation-load MLD is now being investigated that can greatly reduce the MLD computation load.
One example of a prior-art MIMO receiver that uses MLD to perform a CDMA MIMO signal separation process is next shown. FIG. 2 is a block diagram showing the configuration of a MIMO receiver of the prior art. In this case, the number of transmission antennas provided in a transmitter (not shown) is M (where M is an integer equal to or greater than 1), and the number of reception antennas is N (where N is an integer equal to or greater than 1).
Referring to FIG. 2, the MIMO receiver of the prior art includes: reception antennas 911-91N, despreaders 9211-921L, . . . 92N1-92NL, transmission channel estimation unit 93, and MLD unit 94.
Each of despreaders 9211-921L despreads a respective path of each signal that is received at reception antenna 911. Similarly, the received signals of each of the reception antennas are despread for each path by L despreaders, each of despreaders 92N1-92NL despreading the signal received by reception antenna 91N for a respective path. The despread signals that are obtained as a result are provided to MLD unit 94.
In this case, if yn, l is the despread symbol of path l of reception antenna n, despread symbol vector y can be shown as shown in Equation (1).y=[y0,0 . . . y0,L-1y1,0 . . . yN-1,L-1]T  (1)
Transmission channel estimation unit 93 receives as input the signals that have been received at reception antennas 911-91N, uses a known pilot signal that is included in these received signals to estimate for each path the transmission channel estimation value between the transmission and reception antennas. If the transmission channel estimation value of path l between transmission antenna m and reception antenna n is hm, n, l, then transmission channel matrix H can be represented by (N×L) rows and M columns as seen in Equation (2).
                    H        =                  [                                                                      h                                      0                    ,                    0                    ,                    0                                                                                                h                                      1                    ,                    0                    ,                    0                                                                              …                                                              h                                                            M                      -                      1                                        ,                    0                    ,                    0                                                                                                      ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                      h                                      0                    ,                    0                    ,                                          L                      -                      1                                                                                                                    h                                      1                    ,                    0                    ,                                          L                      -                      1                                                                                                  …                                                              h                                                            M                      -                      1                                        ,                    0                    ,                                          L                      -                      1                                                                                                                                            h                                      0                    ,                    1                    ,                    0                                                                                                h                                      1                    ,                    1                    ,                    0                                                                              …                                                              h                                                            M                      -                      1                                        ,                    1                    ,                    0                                                                                                      ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                      h                                      0                    ,                                          N                      -                      1                                        ,                                          L                      -                      1                                                                                                                    h                                      1                    ,                                          N                      -                      1                                        ,                                          L                      -                      1                                                                                                  …                                                              h                                                            M                      -                      1                                        ,                                          N                      -                      1                                        ,                                          L                      -                      1                                                                                                    ]                                    (        2        )            
MLD unit 94 uses transmission channel matrix H that is obtained by transmission channel estimation unit 93 to generate reception replicas for the signals that have been received from all transmission antennas, calculates the error signals between the despread signals from each of despreaders 9211-92NL and the reception replicas, and selects the transmission antenna signal that is most likely.
If the transmission symbol vector s is shown in Equation (3), and noise vector n is shown in Equation (4), transmission channel matrix H can be used to represent despread symbol vector y as shown in Equation (5):
                    s        =                              [                                                                                s                    0                                                                                        s                    1                                                                    …                                                                      s                                          M                      -                      1                                                                                            ]                    T                                    (        3        )                                n        =                              [                                                                                n                                          0                      ,                      0                                                                                        …                                                                      n                                          0                      ,                                              L                        -                        1                                                                                                                                  n                                          1                      ,                      0                                                                                        …                                                                      n                                                                  N                        -                        1                                            ,                                              L                        -                        1                                                                                                                  ]                    T                                    (        4        )                                y        =                  Hs          +          n                                    (        5        )            
In Equation (3), sm shows the transmission symbol of transmission antenna m. In Equation (4), nn, l shows the noise in path l of reception antenna n.
FIG. 3 is a block diagram showing the configuration of MLD unit 94. Referring to FIG. 3, MLD unit 94 includes: transmission symbol candidate generation unit 941, reception replica generation unit 942, error signal calculation unit 943, and bit likelihood calculation unit 944.
Transmission symbol candidate generation unit 941 generates transmission symbol vector s, which is the combination of all transmission antenna symbols, and sends this transmission symbol vector to reception replica generation unit 942.
Reception replica generation unit 942 generates all reception replicas {tilde over (r)}=Hs based on transmission symbol vector s from transmission symbol candidate generation unit 941 and transmission channel matrix H, and sends these transmission replicas to error signal calculation unit 943.
Error signal calculation unit 943 finds the final error signal  based on reception replicas {tilde over (r)} from reception replica generation unit 942 and despread symbols y from despreaders 9211-92NL, and sends this error signal  to bit likelihood calculation unit 944. At this time, error signal calculation unit 943 compares reception replica {tilde over ( )}rn, l and despread symbol yn, l as shown in Equation (6), and then adds each of the error signals as shown in Equation (7) to find the final error signal .
                                          [                                                                                y                                          0                      ,                      0                                                                                                                    ⋮                                                                                                  y                                          0                      ,                                              L                        -                        1                                                                                                                                                              y                                          1                      ,                      0                                                                                                                    ⋮                                                                                                  y                                                                  N                        -                        1                                            ,                                              L                        -                        1                                                                                                                  ]                    ⁢                                                                      ⇔                                                                              ⇔                                                                              ⋮                                                                              ⇔                                                      ⁡                          [                                                                                                                  r                        ~                                                                    0                        ,                        0                                                                                                                                  ⋮                                                                                                                                      r                        ~                                                                    0                        ,                                                  L                          -                          1                                                                                                                                                                                                        r                        ~                                                                    1                        ,                        0                                                                                                                                  ⋮                                                                                                                                      r                        ~                                                                                              N                          -                          1                                                ,                                                  L                          -                          1                                                                                                                                ]                                      =                  H          ⁡                      [                                                                                s                    0                                                                                                                    s                    1                                                                                                ⋮                                                                                                  s                                          M                      -                      1                                                                                            ]                                              (        6        )                                Λ        =                              ∑                          n              =              0                                      N              -              1                                ⁢                                    ∑                              l                =                0                                            L                -                1                                      ⁢                                                                                                y                                          n                      ,                      l                                                        -                                                            r                      ~                                                              n                      ,                      l                                                                                                  2                                                          (        7        )            
Bit likelihood calculation unit 944 receives as input error signals  that correspond to all transmission antenna symbols s and calculates the likelihood for each bit that is transmitted from each transmission antenna. At this time, bit likelihood calculation unit 944 applies the bit likelihood as input to error correction decoder (not shown) (for example, turbo decoder) and restores the information bit sequence. One method for calculating bit likelihood is based on the difference between the minimum error signal of the symbol in which the target bit is +1 and the minimum error signal of the symbol in which the target bit is −1, as described in N. Maeda, K. Higuchi, J. Kawamoto, M. Sawahashi, M. Kimata, and S. Yoshida, “QRM-MLD Combined with MMSE-Based Multipath Interference Canceller for MIMO Multiplexing in Broadband DS-CDMA” (Proc. IEEE PIMRC 2004, pp. 1741-1746, September 2004 (Document 2)).
FIG. 4 is a block diagram showing the configuration of another MIMO receiver of the prior art. The MIMO receiver shown in FIG. 4 has a greatly decreased amount of MLD calculation compared to the device shown in FIG. 2 (see Document 2).
Referring to FIG. 4, the MIMO receiver of the prior art includes: reception antennas 911-91N; despreaders 9211-921L, . . . , 92N1-92NL; transmission channel estimation unit 93; QR decomposition unit 95, QH converter 96; and reduced-calculation-load MLD unit 97.
Reception antennas 911-91N, despreaders 9211-921L, . . . , 92N1-92NL, and transmission channel estimation unit 93 each have the same configuration as FIG. 2.
QR decomposition unit 95 decomposes the transmission channel matrix H that is obtained in transmission channel estimation unit 93 into the product of the Q matrix and R matrix as shown in Equation (8), sends Q to QH converter 96, and sends R to reduced-calculation-load MLD unit 97.
                                                        H              =                            ⁢              QR                                                                          =                            ⁢                              [                                                                                                    q                                                  0                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                                                                                                    q                                                  1                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                                                                            …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                                                                                                                ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          q                                                  0                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                                              q                                                  1                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                      …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                                                                                  q                                                  0                          ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                                                                                                    q                                                  1                          ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                                                                            …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                                                                                                                ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          q                                                  0                          ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                                              q                                                  1                          ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                      …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                              ]                                                                                                      ⁢                              [                                                                                                    r                                                  0                          ,                                                                                                          ⁢                          0                                                                                                                                    r                                                  0                          ,                                                                                                          ⁢                          1                                                                                                            …                                                                                      r                                                  0                          ,                                                                                                          ⁢                                                      M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                                                          0                                                                                      r                                                  1                          ,                                                                                                          ⁢                          1                                                                                                            …                                                                                      r                                                  1                          ,                                                                                                          ⁢                                                      M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                                                          ⋮                                                              0                                                              ⋱                                                              ⋮                                                                                                  0                                                              …                                                              0                                                                                      r                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                                                                                              ]                                                                        (        8        )            
In this case, Q is a unitary matrix of (N×L) rows and M column, each column vector being orthogonal (QHQ=1), and the norm is 1. R is an upper triangular matrix of M rows and M columns.
QH converter 96 multiplies despread symbol vector y with QH to convert y to an orthogonal coordinate system represented by Q. QH converter 96 is of a configuration for realizing computation for multiplying QH by multipliers and adders. Signal vector z after coordinate conversion is represented by Equation (9).
                                                        z              =                            ⁢                                                Q                  H                                ⁢                y                                                                                        =                            ⁢                              [                                                                                                    z                        0                                                                                                                                                z                        1                                                                                                                        ⋮                                                                                                                          z                                                  M                          -                          1                                                                                                                    ]                                                                                        =                            ⁢                              [                                                                                                    q                                                  0                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                  0                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                                            q                                                  0                          ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                  0                          ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                                                                                q                                                  1                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                  1                          ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                                            q                                                  1                          ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                  1                          ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                                                        ⋮                                                              ⋱                                                              ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          0                          ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                                            q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                          1                          ,                                                                                                          ⁢                          0                                                *                                                                                    …                                                                                      q                                                                              M                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      N                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                    ,                                                                                                          ⁢                                                      L                            ⁢                                                                                                                  -                                                                                                                  ⁢                            1                                                                          *                                                                                            ]                                                                                                      ⁢                              [                                                                                                    y                                                  0                          ,                          0                                                                                                                                                ⋮                                                                                                                          y                                                  0                          ,                                                      L                            -                            1                                                                                                                                                                                                  y                                                  1                          ,                          0                                                                                                                                                ⋮                                                                                                                          y                                                                              N                            -                            1                                                    ,                                                      L                            -                            1                                                                                                                                              ]                                                                                        =                            ⁢                                                Q                  H                                ⁡                                  (                                      Hs                    +                    n                                    )                                                                                                        =                            ⁢                                                Q                  H                                ⁡                                  (                                      QRs                    +                    n                                    )                                                                                                        =                            ⁢                                                                    Q                    H                                    ⁢                  QRs                                +                                                      Q                    H                                    ⁢                  n                                                                                                        =                            ⁢                              Rs                +                                                      Q                    H                                    ⁢                  n                                                                                                        =                            ⁢                              Rs                +                                  n                  ′                                                                                        (        9        )            
In this case, noise n′ is noise n projected onto an orthogonal coordinate system represented by Q, and therefore uncorrelated with n at the same power.
Reduced-calculation-load MLD unit 97 uses the R matrix from QR decomposition unit 95 to generate reception replicas for the signals of all transmission antennas, calculates the error signals between the reception replicas and the signal vector z following coordinate conversion, and after cutting back symbol candidates, selects the most likely transmission antenna signal.
FIG. 5 is a block diagram showing the configuration of reduced-calculation-load MLD unit 97. Referring to FIG. 5, reduced-calculation-load MLD unit 97 includes: transmission symbol candidate generation unit 971, reception replica generation unit 972, error signal calculation/symbol candidate reduction unit 973, and bit likelihood calculation unit 974.
As with transmission symbol candidate generation unit 941 of FIG. 3, transmission symbol candidate generation unit 971 generates transmission symbol vector s, which is the combination of all transmission antenna symbols, and sends transmission symbol vector s to reception replica generation unit 972.
Reception replica generation unit 972 generates all reception replicas {tilde over (r)}=Rs from transmission symbol vector s from transmission symbol candidate generation unit 971 and matrix R from QR decomposition unit 95 and sends the reception replicas to error signal calculation/symbol candidate reduction unit 973.
Error signal calculation/symbol candidate reduction unit 973 reduces symbol candidates while finding error signal m from reception replicas {tilde over (r)}m and signal vector zm following coordinate conversion by QH converter 96 over a plurality of stages for the plurality of transmission antennas.
As an example, the reduction of symbol candidates is carried out successively starting from the largest transmission antenna number.
In each stage of the reduction of symbol candidates, error signal calculation/symbol candidate reduction unit 973 compares reception replicas {tilde over (r)}m and signal vectors zm as shown in Equation (10), and finds error signal m as shown in Equation (11). Error signal calculation/symbol candidate reduction unit 973 further reduces symbol candidates by selecting only a prescribed number from the symbol candidates having low error signals m.
                                          [                                                                                z                    0                                                                                                                    z                    1                                                                                                ⋮                                                                                                  z                                          M                      -                      1                                                                                            ]                    ⁢                                                                                                                                                                                                                            ⇔                                                                                                                                          ⇔                                                                                                                                                                                          ⋮                                                                                                                                          ⇔                                                      ⁡                          [                                                                                                                                                                                                                                                                                                            r                                    ~                                                                    0                                                                                                                                                                                                                                                          r                                    ~                                                                    1                                                                                                                                                                                                                                          ⋮                                                                                                                                                                                                        r                        ~                                                                    M                        -                        1                                                                                                        ]                                      =                  R          ⁡                      [                                                                                                                                                                                                                                                  s                                0                                                                                                                                                                                                        s                                1                                                                                                                                                                                                                        ⋮                                                                                                                                                              s                                          M                      -                      1                                                                                            ]                                              (        10        )                                          Λ          m                =                              ∑                          i              =              m                                      M              -              1                                ⁢                                                                                    z                  i                                -                                                      r                    ~                                    i                                                                    2                                              (        11        )            
Bit likelihood calculation unit 974 calculates the likelihood for each bit that is transmitted from each transmission antenna based on the error signals  that correspond to all transmission antenna symbols s that have been finally eliminated.
Nevertheless, the above-described prior art has the following drawbacks.
The MIMO receiver of the prior art shown in FIG. 2 compares the reception replicas that are generated using the transmission channel estimation values of each path with the multipath signals that are separated in despreading to perform a signal separation process by MLD. At this time, the differences in levels of the multipath signals is not taken into consideration, and as a result, when there are differences in the levels of each multipath signal, the multipath interference sustained by each path differs, multipaths having a low level receiving the greatest multipath interference. In a MIMO receiver of the prior art, the influence of multipath interference is not taken into consideration in the calculation of error signals, and as a result, there is a disproportionately large addition of the error signal of multipaths having a low level and a consequent degradation of the signal separation characteristic.
On the other hand, the other MIMO receiver of the prior art shown in FIG. 4, performs the MLD process using signals in which despread signals undergo QH conversion, but because the transmission channel matrix is subjected to direct QR decomposition, the operation is principally equivalent to that of the MIMO receiver of FIG. 2, and is therefore subject to the same problem.